Old Style (O.S.) and New Style (N.S.) indicate dating systems before and after a calendar change, respectively. Usually, they refer to the change from the Julian calendar to the Gregorian calendar as enacted in various European countries between 1582 and 1923. Consequently, the same day can have two dates, one for each calendar used to record it. Before as well as after the legal change, writers used the dual dating convention, to specify a given day by giving its date according to both styles of dating (to ensure that the day concerned was identified unambiguously).

In England, Wales, Ireland, and Britain's American colonies, there were two calendar changes, both in 1752. The first of these was to adjust the start of a new year from 25 March (Lady Day, the Feast of the Annunciation) to 1 January, a change which Scotland had already made in 1600. The second discarded the Julian calendar in favour of the Gregorian calendar, skipping 11 days in the calendar for September 1752 to do so.

For countries such as Russia where no start-of-year adjustment took place, O.S. and N.S. simply indicate the Julian and Gregorian dating systems respectively.

Differences between Julian and Gregorian dates

The need to correct the calendar arose from the realisation that the correct figure for the number of days in a year is not 365.25 (365 days 6 hours) as assumed by the Julian calendar but slightly less (). The Julian calendar therefore has too many leap years. The consequence was that the method for the calculation of the date of Easter, as decided in the 4th century, had drifted from astronomical reality. The Gregorian calendar reform dealt with the accumulated difference between the years 325 and 1582 by skipping 10 days. thereby realigning the ecclesiastical date of the equinox, fixed at 21 March. with the median date of the astronomical March equinox at the time of the First Council of Nicea in 325.

Countries that adopted the Gregorian calendar after 1699 needed to skip an additional day for each subsequent new century that the Julian calendar had added since then. When the British Empire did so in 1752, the gap had grown to eleven days; when Russia did so (as its civil calendar) in 1918, thirteen days needed to be skipped.

It is for this reason that, although the Eastern Orthodox churches and Western Christendom celebrate Christmas on 25 December according to their respective calendars, the day in which they do so differs by 13 days (until March 2100). Eastern Orthodox Christmas is 17 days after the typical date of the northern winter solstice, whereas Western Christmas follows the solstice by four days. (See also Christmas#Date according to Julian calendar.)

Britain and its colonies or possessions

In the Kingdom of Great Britain and its possessions, the Calendar (New Style) Act 1750 (24 Geo. 2. c. 23) introduced two concurrent changes to the calendar. The first, which applied to England, Wales, Ireland and the British colonies, changed the start of the year from 25 March to 1 January, with effect from "the day after 31 December 1751". (Scotland had already made this aspect of the changes, with effect from 1 January 1600.) The second (in effect) adopted the Gregorian calendar in place of the Julian calendar. Thus "New Style" can refer to the start-of-year adjustment, to the adoption of the Gregorian calendar, or to the combination of the two. It was through their use in the 1750 act that the terms and notations "Old Style" and "New Style" came into common usage.

Start-of-year adjustment

When recording British history, it is usual to quote the date as originally recorded at the time of the event, but with the year number adjusted to start on 1 January.e.g. The latter adjustment may be needed because the start of the civil calendar year had not always been 1 January and was altered at different times in different countries. From 1155 to 1752, the civil or legal year in England began on 25 March (ie. Lady Day, following the Annunciation Style);Nørby, Toke. The Perpetual Calendar: What about England? Version 29 February 2000. so for example, the execution of Charles I was recorded at the time in Parliament as happening on 30 January 1648. In newer English-language texts, this date is usually shown as "30 January 1649",Death warrant of Charles I web page of the UK National Archives. A demonstration of New Style, meaning Julian calendar with a start of year adjustment. which is how chroniclers in Scotland would have recorded it. (The corresponding date in the Gregorian calendar was 9 February 1649, but this is not customarily used).

The O.S./N.S. designation is particularly relevant for dates which fall between the start of the "historical year" (1 January) and the legal start date, where different. This was 25 March in England, Wales, Ireland and the colonies until 1752, and until 1600 in Scotland. Thereafter, in both cases, it became 1 January.

In Britain, 1 January was celebrated as the New Year festival from as early as the 13th century, despite the recorded (civil) year not incrementing until 25 March, but the "year starting 25th March was called the Civil or Legal Year, although the phrase Old Style was more commonly used". To reduce misunderstandings about the date, it was normal even in semi-official documents such as parish registers to place a statutory new-year heading after 24 March (for example "1661") and another heading from the end of the following December, 1661/62, a form of dual dating to indicate that in the following twelve weeks or so, the year was 1661 Old Style but 1662 New Style.Spathaky, Mike Old Style and New Style Dates and the change to the Gregorian Calendar. "An oblique stroke is by far the most usual indicator, but sometimes the alternative final figures of the year are written above and below a horizontal line, as in a fraction, thus: . Very occasionally a hyphen is used, as ." Some more modern sources, often more academic ones (e.g. the History of Parliament) also use the 1661/62 style for the period between 1 January and 24 March for years before the introduction of the New Style calendar in England.See for example this biographical entry:

Table + Examples of Old Style (O.S.) and New Style (N.S.) Dates for England and Colonies Event Old Style Date Historical Julian Date New Style Date Day Drift Explanation Walter Raleigh's Virginia charter 24 March 1583 24 March 1584 3 April 1584 10 days Signed on the final day of legal year 1583 O.S. A 10-day Gregorian drift applies. Spanish Armada Battle of Gravelines 29 July 1588 29 July 1588 8 August 1588 10 days July falls after 25 March, so no year shift occurs. A 10-day Gregorian calendar drift applies relative to Spanish/Continental records. Death of Elizabeth I 24 March 1602 24 March 1603 3 April 1603 10 days Elizabeth died on the last day of the year of 1602 O.S. English settlers arrive at Cape Henry, Virginia 26 April 1607 26 April 1607 6 May 1607 10 days behind No year adjustment is needed because the month April falls after 25 March. The Julian calendar was 10 days behind the Gregorian calendar. Indian massacre of 1622 22 March 1621 22 March 1622 1 April 1622 10 days behind The year number was adjusted because the event occurred before 25 March. Calendars were 10 days apart. Birth of George Washington 11 February 1731 11 February 1732 22 February 1732 11 days The year incremented because the date falls before 25 March. The drift became 11 days because 1700 was a leap year in the Julian calendar, but not in the Gregorian. The 1752 Calendar Transition 2 September 1752 2 September 1752 14 September 1752 11 days skipped Wednesday, 2 September (O.S.) was immediately followed by Thursday, 14 September (N.S.) to discard the 11 accumulated days of drift (aligning with the Gregorian calendar).

Russia

The Gregorian calendar was implemented in Russia on by dropping the Julian dates of , pursuant to a Sovnarkom decree signed (Julian) by Vladimir Lenin. The decree required that the Julian date was to be written in parentheses after the Gregorian date, until

It is common in English-language publications to use the familiar Old Style or New Style terms to discuss events and personalities in other countries, especially with reference to the Russian Empire and the very beginning of Soviet Russia. For example, in the article "The October (November) Revolution", the Encyclopædia Britannica uses the format of "25 October (7 November, New Style)" to describe the date of the start of the revolution.

Other notations

Latin notation: st.v. and st.n. The Latin equivalents, which are used in many languages, are, on the one hand, stili veteris (genitive) or stilo vetere (ablative), abbreviated st.v., and meaning ; and, on the other, stili novi or stilo novo, abbreviated st.n. and meaning The Latin abbreviations may be capitalised differently by different users, e.g., St.n. or St.N. for stili novi. There are equivalents for these terms in other languages as well, such as the German a.St. ("alter Stil" for O.S.).

Transposition of historical event dates and possible date conflicts

Usually, the mapping of New Style dates onto Old Style dates with a start-of-year adjustment works well with little confusion for events before the introduction of the Gregorian calendar. For example, the Battle of Agincourt is well known to have been fought on 25 October 1415, which is Saint Crispin's Day. However, for the period between the first introduction of the Gregorian calendar on 15 October 1582 and its introduction in Britain on 14 September 1752, there can be considerable confusion between events in Continental Western Europe and in British domains. Events in Continental Western Europe are usually reported in English-language histories by using the Gregorian calendar. For example, the Battle of Blenheim is always given as 13 August 1704. However, confusion occurs when an event involves both. For example, William III of England arrived at Brixham in England on 5 November (Julian calendar), after he had set sail from the Netherlands on 11 November (Gregorian calendar) 1688..

The Battle of the Boyne in Ireland took place a few months later on 1 July 1690 (Julian calendar). That maps to 11 July (Gregorian calendar), conveniently close to the Julian date of the subsequent (and more decisive) Battle of Aughrim on 12 July 1691 (Julian). The latter battle was commemorated annually throughout the 18th century on 12 July, following the usual historical convention of commemorating events of that period within Great Britain and Ireland by mapping the Julian date directly onto the modern Gregorian calendar date (as happens, for example, with Guy Fawkes Night on 5 November). The Battle of the Boyne was commemorated with smaller parades on 1 July. However, both events were combined in the late 18th century, and continue to be celebrated as "The Twelfth".